Polyethylene film, laminate and package using the same

The present invention relates to a polyethylene film, and more particularly, to a single-layer polyethylene film having different physical properties on its front and back sides, and a package using the polyethylene film.

The present invention also relates to a laminate, and more particularly, to a laminate comprising a polyethylene film substrate that is irradiated with an electron beam on its both sides and a polyethylene film layer that is not irradiated with an electron beam on its at least one side, and a package using the laminate.

Films made of polyethylene have moderate flexibility, are excellent in, for example, transparency, moisture resistance and chemical resistance, and are inexpensive, and thus they are used for various packaging materials. In particular, since the melting point of polyethylene, which varies in some degree depending on its kind, is generally about 100 to 140° C., polyethylene is generally used as a sealant film in the field of packaging materials.

On the other hand, polyethylene is inferior in heat resistance and also insufficient in strength as compared with other thermoplastic resins. Thus, when polyethylene is used as a packaging material, it is used as a lamination film obtained by laminating a resin film, such as a polyester film and a nylon film, excellent in heat resistance and strength and a polyethylene film. A package is manufactured by heat-sealing the edge of the lamination film so that the polyethylene film side is inside of the package (for example, Japanese Laid-open Patent Application (Kokai) No. 2005-104525).

In recent years, there has been an increasing demand for construction of a recycling-oriented society, and along with this, recycling of packaging materials has been attempted. However, a lamination film obtained by laminating different kinds of resin films as described above has had a problem that it is not suitable for recycling because of the difficulty in being separated for each resin type.

The present inventors have now found that irradiation of an electron beam to a polyethylene film can occur curing or crosslinking of polyethylene near the film surface irradiated with the electron beam.

In addition, it was found that use of a polyethylene film irradiated with an electron beam, using only a polyethylene film instead of a laminate obtained by laminating different resin films conventionally used in a package, can provide a package suitable for recycling as well as having high heat resistance and strength. The present invention is based on this finding.

Thus, an object of the present invention is to provide a polyethylene film and a laminate which can be used to produce a package having high heat resistance, strength and recycling suitability in place of a lamination film conventionally used in a package. Another object of the present invention is to provide a package using such a polyethylene film and a laminate.

In one embodiment, the polyethylene film of the present invention is irradiated with an electron beam on its only one side and comprises polyethylene and a crosslinking agent, wherein the crosslink density of the polyethylene is different between the side irradiated with the electron beam and the other side not irradiated with the electron beam.

In one embodiment, the polyethylene film of the present invention is irradiated with an electron beam on its only one side and comprises a low-density polyethylene having a density of 0.91 g/cmor less, wherein the crosslink density of the polyethylene is different between the side irradiated with the electron beam and the other side not irradiated with the electron beam.

In one embodiment, the polyethylene film of the present invention is irradiated with an electron beam on its only one side and comprises polyethylene and a light stabilizer, wherein the crosslink density of the polyethylene is different between the side irradiated with the electron beam and the other side not irradiated with the electron beam.

In one embodiment, the laminate of the present invention comprises a polyethylene film substrate and a polyethylene film layer, wherein the polyethylene film substrate is irradiated with an electron beam on its both sides and comprises polyethylene and a crosslinking agent, and wherein the polyethylene film layer is not irradiated with an electron beam on at least the side opposite to the side of the polyethylene film substrate and has heat sealability.

In one embodiment, the laminate of the present invention comprises a polyethylene film substrate and a polyethylene film layer, wherein the polyethylene film substrate is irradiated with an electron beam on its both sides and comprises a low-density polyethylene having a density of 0.91 g/cmor less, and wherein the polyethylene film layer is not irradiated with an electron beam on at least the side opposite to the side of the polyethylene film substrate and has heat sealability.

In one embodiment, the laminate of the present invention comprises a polyethylene film substrate and a polyethylene film layer, wherein the polyethylene film substrate is irradiated with an electron beam on its both sides and comprises polyethylene and a light stabilizer, and wherein the polyethylene film layer is not irradiated with an electron beam on at least the side opposite to the side of the polyethylene film substrate and has heat sealability.

According to the present invention, irradiation of an electron beam to a polyethylene film can occur curing or crosslinking of polyethylene near the film surface irradiated with the electron beam. As a result, this can provide a single-layer polyethylene film in which the crosslink densities of polyethylene on the front and back sides are different. Since the polyethylene film surface irradiated with an electron beam and thereby having a higher crosslink density than that of usual polyethylene has improved heat resistance and strength, the surface can satisfy the physical properties required as the outer layer of package. On the other hand, since the other surface to which the electron beam is not irradiated has the same physical properties as those of the conventional polyethylene film, the surface can maintain the sealant property and flexibility required as the inner layer of package. Therefore, by using such a polyethylene film, a package can be manufactured using only a single-layer polyethylene film instead of a lamination film used for a package.

In addition, according to the present invention, irradiation of an electron beam to a polyethylene film substrate constituting a laminate can occur curing or crosslinking of polyethylene in the film substrate irradiated with the electron beam. Since the surface of the polyethylene film substrate irradiated with an electron beam and thereby having a higher crosslink density than that of usual polyethylene has improved heat resistance and strength, the surface can satisfy the physical properties required as the outer layer of package. Further, since the laminate according to the present invention comprises a polyethylene film layer that is not irradiated with an electron beam on at least its one side, or maintaining heat-sealability and flexibility, it can be used to prepare a package.

Polyethylene Film

The polyethylene film according to the present invention will be described with reference to the drawings.is a schematic cross-sectional view of a polyethylene film according to one embodiment of the present invention.

The polyethylene filmis irradiated with an electron beam on its only one side, and the crosslink density of the polyethylene is different between the sideirradiated with an electron beam and the other sidenot irradiated with an electron beam.

The reason why the crosslink density of polyethylene varies depending on the presence or absence of an electron irradiation is not clear but is considered as follows. When polyethylene is irradiated with an electron beam, carbon-hydrogen bonds in the polyethylene near the irradiated film surface are cleaved, and radicals are generated at the ends of the cleaved bonds. The generated radical is considered to be brought into contact with other polyethylene molecular chain due to the molecular motion of the molecular chain and extract a hydrogen atom to bond with a carbon atom in the polyethylene molecular chain, thereby forming a crosslinked structure.

In general, polyethylene films tend to contract when heated, and as the crosslink density increases, the dimensional stability tends to be improved. Therefore, polyethylene films having different crosslink densities on the front and back sides are curled like a bimetal when heated. Therefore, as a simple method for confirming that crosslink densities are different between the front and back sides of the polyethylene film, it can be confirmed by heating the obtained polyethylene film.

The crosslink density can also be determined by a method utilizing the fact that the crosslinking moiety does not dissolve in the solvent, comprising immersing the polyethylene film in an organic solvent such as methyl ethyl ketone, drying the insoluble film remaining without being dissolved, measuring the mass and calculating the gel fraction from the masses of the polyethylene film before dissolution and the insoluble film after drying. Specifically, X g of a polyethylene film is first wrapped with Y g of a stainless steel wire mesh, heated and dipped in a solvent to obtain the polyethylene film wrapped with the stainless steel wire mesh. Next, after vacuum-drying, the mass (Z g) of the polyethylene film wrapped with the stainless steel wire mesh after drying is measured. Gel fraction can be determined from the following formula (1):Gel fraction (% by mass)=()/100  (1)

The gel fraction of the polyethylene film is preferably 20 to 80%, more preferably 30 to 80%, still more preferably 40 to 80%.

The polyethylene film according to the present invention comprises polyethylene. Examples of the polyethylene that can be used include those obtained by mixing one or two or more kinds of polyethylenes having different density and branching, such as high-density polyethylene (HDPE), medium-density polyethylene (MDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE). In general, high-density polyethylene refers to polyethylene having a density of 0.940 g/cmor more, medium-density polyethylene refers to polyethylene having a density of 0.925 to 0.940 g/cm, and low-density polyethylene refers to polyethylene having a density of less than 0.925 g/cm.

In one embodiment, the polyethylene film of the present invention comprises a low-density polyethylene having a density of 0.91 g/cmor less.

This makes it possible to realize a higher crosslink density and improve the heat resistance of the polyethylene film. The polyethylene film comprises a low-density polyethylene having a density more preferably of 0.91 g/cmor less and 0.89 g/cmor more, still more preferably of 0.91 g/cmor less and 0.895 g/cmor more.

The above-mentioned low-density polyethylene may be a linear chain or a branched chain, but preferably is a linear chain since it can realize a higher crosslink density.

The content of polyethylene having a density of 0.91 g/cmor less in the polyethylene film is preferably 10% by mass or more and 100% by mass or less, and more preferably 20% by mass or more and 70% by mass or less.

Polyethylenes having different densities and branchings as described above can be obtained by appropriately selecting a polymerization method. For example, the polymerization method is preferably carried out in one stage or in multiple stages of two or more stages, by either one of gas phase polymerization, slurry polymerization, solution polymerization, and high-pressure ion polymerization, using a multi-site catalyst such as a Ziegler-Natta catalyst or a single-site catalyst such as a metallocene catalyst as a polymerization catalyst.

The above-mentioned single-site catalyst refers to a catalyst capable of forming a uniform active species, and is usually adjusted by bringing a metallocene-type or nonmetallocene-type transition metal compound into contact with an activating promoter. Since the single-site catalyst has a uniform active site structure as compared with that of multi-site catalyst, a polymer having a structure with high molecular weight and high homogeneity can be preferably polymerized. A single-site catalyst which is particularly preferably used is a metallocene catalyst. The metallocene catalyst is a catalyst containing each of catalyst components comprising: a transition-metal compound of Group IV of the periodic table containing a ligand having a cyclopentadienyl skeleton; a promoter; an organometallic compound, if necessary; and a carrier.

In the above-mentioned transition-metal compound of Group IV of the periodic table containing a ligand having a cyclopentadienyl skeleton, the cyclopentadienyl skeleton is, for example, a cyclopentadienyl group or a substituted cyclopentadienyl group. Examples of the substituted cyclopentadienyl group include those having at least one substituent selected from C-Chydrocarbon, silyl, silyl-substituted alkyl, silyl-substituted aryl, cyano, cyanoalkyl, cyanoaryl, halogen, haloalkyl, and halosilyl. The substituted cyclopentadienyl group may have two or more substituents, and the substituents may together form a ring, such as an indenyl ring, a fluorenyl ring, an azulenyl ring, or a hydrogenated product thereof. The rings formed by bonding of the substituents may each further have a substituent.

For the transition-metal compound of Group IV of the periodic table containing a ligand having a cyclopentadienyl skeleton, examples of the transition metal include zirconium, titanium and hafnium, and among them, zirconium and hafnium are preferred. The transition metal compound usually has two ligands having a cyclopentadienyl skeleton and each of the ligands having cyclopentadienyl skeleton are preferably bound to each other by a crosslinking group. Examples of the crosslinking group include C-Calkylene; silylene; substituted silylene such as dialkylsilylene and diarylsilylene; and substituted germylene such as dialkylgermylene and diaryl germylene. Among them, substituted silylene is preferred. For the above-mentioned transition-metal compound of Group IV of the periodic table containing a ligand having a cyclopentadienyl skeleton, the catalyst component may be a single or a mixture of two or more of them.

The promoter refers to those which can make the transition-metal compound of Group IV of the periodic table effective as a polymerization catalyst or can equalize an ionic charge in a catalytically activated state. Examples of the promoter include benzene-soluble aluminoxane of organoaluminum oxy-compound or benzene-insoluble organoaluminum oxy-compound; ion-exchangeable layered silicate; boron compounds; ionic compounds composed of a cation containing or not containing an active hydrogen group and a noncoordinating anion; lanthanoid salts such as lanthanum oxide; tin oxide; and phenoxy compounds containing a fluoro group.

The transition-metal compound of Group IV of the periodic table containing a ligand having a cyclopentadienyl skeleton may be used by supporting it on a carrier which is an inorganic or organic compound. As the carrier, porous oxides of inorganic or organic compounds are preferable, and specific examples thereof include an ion-exchangeable layered silicate such as montmorillonite, SiO, AlO, MgO, ZrO, TiO, BO, CaO, ZnO, BaO, ThOor a mixture thereof. Examples of the organometallic compound used as necessary include organoaluminum compound, organomagnesium compound, and organozinc compound. Among them, organoaluminum is preferably used.

Copolymers of ethylene and other monomers may also be used. Examples of the ethylene copolymer include copolymers comprising ethylene and a C-Cα-olefin. Examples of the C-Cα-olefin include propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene, 1-eicosene, 3-methyl-1-butene, 4-methyl-1-pentene, and 6-methyl-1-heptene. A copolymer with vinyl acetate or an acrylate ester may be used as long as it does not impair the object of the present invention.

In the present invention, polyethylene obtained by using as a raw material a biomass-derived ethylene in place of a fossil fuel-derived ethylene may be used. Since such a biomass-derived polyethylene is a carbon-neutral material, a more environmentally-friendly package can be obtained. Such a biomass-derived polyethylene can be produced by the method described in, for example, Japanese Laid-open Patent Application (Kokai) No. 2013-177531. Commercially available biomass-derived polyethylene (e.g., Green PE commercially available from Braskem) may be used.

In one embodiment, the polyethylene film of the present invention comprises a crosslinking agent. The polyethylene film comprises a crosslinking agent in addition to polyethylene, so that a higher crosslink density of polyethylene film can be realized and the heat resistance can be improved.

Examples of the crosslinking agent include styrene elastomers such as styrene-polyisoprene elastomer, styrene-polybutadiene elastomer, styrene-polyisoprene-butadiene random copolymer; ethylene-acrylate copolymers such as ethylene-methyl acrylate copolymer, ethylene-ethyl acrylate copolymer, ethylene-butyl acrylate copolymer; and ethylene-acrylic ester-glycidyl methacrylate.

The content of the crosslinking agent in the polyethylene film is preferably 1 to 49% by mass, more preferably 10 to 40% by mass, still more preferably 15 to 35% by mass. When the content of the crosslinking agent is within the above numerical range, the heat resistance and strength of the polyethylene film can be further improved.

In one embodiment, the polyethylene film of the present invention comprises a light stabilizer. The polyethylene film comprises a light stabilizer, so that degradation of the polyethylene film over time can be prevented.

Examples of the light stabilizer include antioxidants such as phenolic antioxidant, amine antioxidant, phosphate antioxidant, sulfur antioxidant, hindered amine antioxidant and hydroxylamine antioxidant; and ultraviolet absorbers such as benzotriazole ultraviolet absorber, triazine ultraviolet absorber and benzophenone ultraviolet absorber. Among them, the antioxidants are preferably used since they hardly inhibit the crosslinking reaction initiated by electron irradiation to the polyethylene film.

As the antioxidant, a primary antioxidant for capturing generated radicals and a secondary antioxidant for decomposing hydroperoxide generated from radicals are preferably used in combination. Alternatively, an antioxidant having both functions of a primary antioxidant and a secondary antioxidant may be used.

Examples of the primary antioxidant include phenolic antioxidant, amine antioxidant and hindered amine antioxidant; and examples of the secondary antioxidant include phosphorus antioxidant and sulfur antioxidant; and examples of the antioxidant having both functions of a primary antioxidant and a secondary antioxidant include hydroxylamine antioxidant.

Hydroxylamine antioxidant and phosphorus antioxidant are also preferred since they can prevent coloring of the polyethylene film.

The content of the light stabilizer in the polyethylene film is preferably 0.01% by mass or more and 10% by mass or less, more preferably 0.1% by mass or more and 10% by mass or less, still more preferably 0.1% by mass or more and 8% by mass or less.

When the content of the light stabilizer is within the above numerical range, crosslinking reaction of polyethylene in the polyethylene film can be satisfactorily carried out and degradation of the film over time can be prevented.

The polyethylene film may contain various plastic compounding agents, additives and the like for the purpose of improving or modifying, for example, processability, heat resistance, weather resistance, mechanical properties, dimensional stability, antioxidant properties, slip properties, mold releasability, fire retardant properties, antifungal properties, electrical properties and strength of the film, and the amount to be added can be varied depending on the purpose, from very small amount to several tens of percent. Typical additives include, for example, fillers, reinforcing agents, antistatic agents, pigments and modifier resins.

The thickness of the polyethylene film may vary depending on its use, and is usually about 5 μm to 200 μm, preferably about 5 μm to 100 μm. The film thickness can be appropriately adjusted depending on, for example, the screw rotation speed of the melt extruder and the rotation speed of the cooling roll.

The polyethylene film can be obtained by melting a resin composition comprising at least the above-mentioned polyethylene and film-forming it by a melt extrusion molding method such as inflation molding or T-die molding. The polyethylene film can be molded, for example, by feeding the resin composition to a melt extruder heated to a temperature equal to or higher than the melting point (Tm) of the polyethylene to a temperature of Tm+120° C. to melt it and extruding it in a cylindrical shape from a die such as a circular die, and then sending air to the extruded cylindrical object to form a bubble and pressurizing it with a roller.